I'm changing things up a bit for this guide. Because this encounter is so complicated, I'll discuss strategy and tips for each phase in turn.

Ragnaros, phase 1 Your raid will be formed in a loose semicircle or U-shape around Ragnaros, who remains immobile throughout the fight. Tanks will be on one end, melee the other, and ranged and healers hang out in the middle.

There are two knockback effects in this phase -- Wrath of Ragnaros, which targets a random player and everyone within 6 yards of him, and Hand of Ragnaros, which targets everyone. You'll want to be aware of your positioning at all times, to avoid an untimely death.

Other than knockbacks, the main mechanic you will dodge in this phase is Sulfuras Smash. Every 40 seconds or so, Ragnaros will mark a spot on the platform, wait a few seconds, and slam down his hammer. Move, obviously. The spot struck by the hammer will spawn three Lava Waves, which travel left, right, and back (away from Rag). Dodging these shouldn't be a big issue; just move in closer to Rag as necessary, to let the Wave pass behind you. Note that you really don't want to have the Smash behind you; getting knocked back into it is not fun.

Magma Traps will also spawn fairly frequently throughout this phase. They have to be detonated in a controlled manner; they don't despawn, and keeping the room clear is essential for later phases. Detonating a trap hits raid members for roughly 50% of their HP and knocks anyone nearby very high in the air. We can do trap detonation if we have to (due to our reduced falling damage), but the preferred choice is a mage/priest/lock with their Slowfall/Levitate/Teleport, respectively.

Don't worry; the worst is yet to come.

Ragnaros, phase 1.5

At 70% HP, Rag will plant Sulfuras in the platform (again, move) and take a lava nap for a bit. During this, eight Sons of Flame will spawn and move toward the hammer, causing a wipe if they reach it. Sons of Flame move very fast at high HP, so the emphasis will be on quick damage and stopping their movement via stuns.

On 10-man, you'll have to solo kill one Son. Thankfully, they have very low HP (165k), so you can safely ignore the stun and just burn it. A Mangle, Rake, Tiger's Fury, Shredx2, Ferocious Bite cycle should just about kill one every time. Once killed, use Feral Charge and Ravage! to help out someone else. If you get a close add, you may want to start with a Bash (outlined below).

On 25-man, you'll be teamed with someone else, but you'll have to burn through much more health (665k). Instead of trying to speed kill, you'll need to use control. If you're assigned as the controller in your duo, the best strategy is to start in Bear.

When your Son spawns, Enrage and hit Bash, then go Cat and build 5 CPs via the normal rotation. Instead of Ripping, though, you'll want to use Maim as the Bear stun wears off. Between those two stuns, you should have plenty of time (with your partner) to get the add below 50% before he moves too much. It's critical to get all the adds below 50%, which will slow their movement rate and give you time to finish them off before reaching the hammer.

Ragnaros, phase 2

Here's where things start to get tricky. Continue to dodge the Sulfuras Smashes, and watch out for the two new mechanics, Molten Seeds and Engulfing Flame. Engulfing Flame is fairly trivial to deal with; Rag will light either the close-range, middle, or long-range areas of his platform on fire. The only one that should be a problem is the close-range one, and you can simply back out.

Molten Seeds, however, are much more challenging. Every minute, a Seed will spawn beneath every raid member (or 20 members in 25-man), doing 55k damage to that player and everyone else within 6 yards. Furthermore, the Seeds eventually explode in a Molten Inferno, doing quite a bit of damage to players still near them, and spawn Molten Elementals that must be AOEed down.

This part is much easier on 10-man, as you have much more room to spread out. On 25-man, expect to undergo some creative positioning with the melee group to keep everyone in melee range but still spread. Once the Seeds spawn, you'll group up in a predetermined spot (typically middle front of the platform seems to work well), eat any Inferno damage, and AOE down the Elementals. Your position will adjust slightly depending on whether Sulfuras Smash or Engulfing Flame will follow the Seeds. (See the WoW Insider Ready Check guide for more on this.) At 40%, phase 2.5 starts.

Ragnaros, phase 2.5

This phase works similarly to the first transition phase, with one major addition: Lava Scions. These two adds will be picked up by your tanks and held, to be killed in phase 3. However, Lava Scions can cast an annoying debuff, Blazing Heat, which leaves a trail that does damage and heals Sons. Players with Blazing Heat will have to get to the edges of the platform ASAP, which totally screws up your organized Son-killing. You'll have to remain very aware of your surroundings and help others (or call for help on your assigned target) as needed. If possible, delay killing the last Son as long as possible to get DPS on the Scions.

Ragnaros, phase 3

Now things get real. The Lava Scions are still up and need to be killed ASAP, to get rid of the Blazing Heat debuffs. Rag keeps using Smash and Engulfing Flame, and he adds one killer new ability: Living Meteor. Periodically, Rag will summon a Meteor that will fixate on one target, chasing it. It doesn't move especially fast, but if anyone comes within 5 yards of a Meteor, it explodes, killing everyone within 8 yards. Happily, a single cast or attack will knock back the meteor and force it to fixate on a new target.

There's not much point to kiting the meteors as melee, so if one targets you, hit it with a Faerie Fire to back it off and continue to DPS. As meteors continue to spawn, things get crazy. Feel free to use FF or Moonfire to keep meteors away from the melee entirely; just be careful to not knock one back into the rest of the raid. Get Ragnaros' health down to 10% and you win!

Advanced tips

Do not use Feral Charge on Rag at any time, unless you like swimming in magma. (Skull Bash doesn't work to counter the knockback, either, though you can Skull Bash then bear Charge back in if knocked out of melee range.)